GPS is a navigation and position service offering worldwide, 24 hour coverage. GPS includes satellites to broadcast GPS satellite signals, stations to monitor and control the satellites, and receivers. GPS receivers demodulate the satellite signals to calculate a range for each satellite from which signals are being received ("in view"), and from that range compute the location of the receiver.
GPS satellites broadcast two codes. Each of these codes, as part of the standard GPS signal structure, contain and are modulated with information from which the location of the receiver can be derived. The two codes are: (1) Precision or Protected code ("P code"); and (2) Coarse Acquisition code ("C/A code"). Although P code can be encrypted by an additional code to prevent unauthorized use, C/A code is freely available.
Each GPS satellite broadcasts on two frequency bands ("carrier bands"), one centered on 1575.42 megahertz, the "L1" frequency band, and the other centered on 1227.6 megahertz, the "L2" frequency band. Each frequency band includes the P code, but only the L1 band includes the C/A code. Each GPS satellite is assigned a unique C/A code and a unique P code. The unique codes enable a GPS receiver to distinguish the GPS signals being received from each other.
To enhance system performance, the standard GPS signal structure can be augmented with split spectrum modulation. The power distribution of a split spectrum modulated signal is substantially different from that of a standard spectrum signal. That is, the power distribution of the standard GPS signal largely surrounds the L1 carrier center frequency, whereas the power distribution of a split spectrum signal is concentrated away from and on opposite sides of the center frequency. This separation of power distribution concentrations may be used advantageously to improve jamming performance, and is described in connection with at least one of the embodiments of the invention described below. Split spectrum modulation can be added to transmissions from satellites or ground-based sources ("pseudolites") in the same frequency band used by the standard GPS signal structure.